se_m <- function(x) sd(x) / sqrt(length(x))
se_m(1:20)  # 1.322876
se_m(c(NA, 1:20))
	
# !is.na version
se_m <- function(x) sd(x[!is.na(x)]) / sqrt(length(x[!is.na(x)]))
se_m(1:20)
se_m(c(NA, 1:20))